The range of the use of optical fiber has been remarkably increased in recent years, particularly in the field of communications, because the information-transmitting capacity of optical fiber is large and optical fiber is relatively free from intervention from outside. Since optical fiber is used in the field of communication, it is generally made of glass. However, glass fiber is brittle and is chemically attacked by water vapour, and accordingly, it is easily broken and is handled with difficulty. Therefore, the surface of optical glass fiber has hitherto been coated with a resin.
As a resinous coating material for such a purpose, epoxy resin, urethane resin and the like have been hitherto used, however, since they are poor in productivity because of the necessity of a long term for curing them and they lack pliability, they have a disadvantage in the transmission specificity of the optical glass fiber coated therewith is reduced by side pressure. For the purpose of improving the above-mentioned disadvantage, the compositions containing urethane acrylate, which are curable by ultraviolet rays, have been actively investigated recently, and such a composition for optical glass fiber and a method for forming a coating have been proposed, for instance, in Japanese Patent Applications Laid-Open (KOKAI) No. 58-223638 (1983) and No. 59-170154 (1984).
In these patent applications, a primary coating of a very low modulus has been used for solving the above-mentioned problem, and a considerable success has been obtained thereby. However, for providing the low modulus, the strength and hardness necessitated to the coating on the surface of glass have been sacrificed, and accordingly, it is desirable to form a top coating onto the primary coating. Compositions for the top coating, which are curable by ultraviolet rays, have been investigated. For instance, a composition for a top coating curable by ultraviolet rays is proposed in Japanese Patent Application Laid-Open (KOKAI) No. 59-170155 (1984).